The Boost Iterator Library contains two parts. The first
is a system of concepts which extend the C++ standard
iterator requirements. The second is a framework of
components for building iterators based on these
extended concepts and includes several useful iterator
adaptors. The extended iterator concepts have been
carefully designed so that old-style iterators
can fit in the new concepts and so that new-style
iterators will be compatible with old-style algorithms,
though algorithms may need to be updated if they want to
take full advantage of the new-style iterator
capabilities. Several components of this library have
been accepted into the C++ standard technical report.
The components of the Boost Iterator Library replace the
older Boost Iterator Adaptor Library.

The iterator categories defined in C++98 are extremely limiting
because they bind together two orthogonal concepts: traversal and
element access. For example, because a random access iterator is
required to return a reference (and not a proxy) when dereferenced,
it is impossible to capture the capabilities of
vector<bool>::iterator using the C++98 categories. This is the
infamous "vector<bool> is not a container, and its iterators
aren't random access iterators", debacle about which Herb Sutter
wrote two papers for the standards comittee (n1185 and n1211),
and a Guru of the Week. New-style iterators go well beyond
patching up vector<bool>, though: there are lots of other
iterators already in use which can't be adequately represented by
the existing concepts. For details about the new iterator
concepts, see our

Writing standard-conforming iterators is tricky, but the need comes
up often. In order to ease the implementation of new iterators,
the Boost.Iterator library provides the iterator_facade class template,
which implements many useful defaults and compile-time checks
designed to help the iterator author ensure that his iterator is
correct.

It is also common to define a new iterator that is similar to some
underlying iterator or iterator-like type, but that modifies some
aspect of the underlying type's behavior. For that purpose, the
library supplies the iterator_adaptor class template, which is specially
designed to take advantage of as much of the underlying type's
behavior as possible.

The documentation for these two classes can be found at the following
web pages:

transform_iterator (PDF): an iterator over elements which are the result of
applying some functional transformation to the elements of an
underlying sequence. This component also replaces the old
projection_iterator_adaptor.

zip_iterator (PDF): an iterator over tuples of the elements at corresponding
positions of heterogeneous underlying iterators.

If you have been using the old Boost Iterator Adaptor library to
implement iterators, you probably wrote a Policies class which
captures the core operations of your iterator. In the new library
design, you'll move those same core operations into the body of the
iterator class itself. If you were writing a family of iterators,
you probably wrote a type generator to build the
iterator_adaptor specialization you needed; in the new library
design you don't need a type generator (though may want to keep it
around as a compatibility aid for older code) because, due to the
use of the Curiously Recurring Template Pattern (CRTP) [Cop95],
you can now define the iterator class yourself and acquire
functionality through inheritance from iterator_facade or
iterator_adaptor. As a result, you also get much finer control
over how your iterator works: you can add additional constructors,
or even override the iterator functionality provided by the
library.

If you're looking for the old projection_iterator component,
its functionality has been merged into transform_iterator: as
long as the function object's result_type (or the Reference
template argument, if explicitly specified) is a true reference
type, transform_iterator will behave like
projection_iterator used to.

In 2000 Dave Abrahams was writing an iterator for a container of
pointers, which would access the pointed-to elements when
dereferenced. Naturally, being a library writer, he decided to
generalize the idea and the Boost Iterator Adaptor library was born.
Dave was inspired by some writings of Andrei Alexandrescu and chose a
policy based design (though he probably didn't capture Andrei's idea
very well - there was only one policy class for all the iterator's
orthogonal properties). Soon Jeremy Siek realized he would need the
library and they worked together to produce a "Boostified" version,
which was reviewed and accepted into the library. They wrote a paper
and made several important revisions of the code.

Eventually, several shortcomings of the older library began to make
the need for a rewrite apparent. Dave and Jeremy started working
at the Santa Cruz C++ committee meeting in 2002, and had quickly
generated a working prototype. At the urging of Mat Marcus, they
decided to use the GenVoca/CRTP pattern approach, and moved the
policies into the iterator class itself. Thomas Witt expressed
interest and became the voice of strict compile-time checking for
the project, adding uses of the SFINAE technique to eliminate false
converting constructors and operators from the overload set. He
also recognized the need for a separate iterator_facade, and
factored it out of iterator_adaptor. Finally, after a
near-complete rewrite of the prototype, they came up with the
library you see today.